Nuclear reactor fuel elements

ABSTRACT

A method of producing coated particles of uranium dicarbide wherein a layer of pyrolytic carbon is deposited on uranium carbide particles of composition from UC1.0 to UC1.6 at a temperature (e.g., 1000* C. to 1400* C.) such that little reaction occurs between the particles and the applied carbon coating followed by application of an outer coating of high density fission product retaining material on the carbon coated particles at a temperature (e.g., above 1450* C.) such that reaction takes place between the initial coating of pyrolytic carbon and the uranium carbide particles to form uranium dicarbide (UC1.85). A gap is produced between the particles and the coating on the particles.

United States Patent [72] Inventors John VincentShennan Preston; Lionel Houston Ford, St. Annes-on-Sea, both of England [21] App]. No. 713,287

[22] Filed Mar. 15, 1968 [45] Patented Nov. 2, 1971 [73] Assignee United Kingdom Atomic Energy Authority London, England [32] Priority Mar. 22, 1967 [33] Great Britain [54] NUCLEAR REACTOR FUEL ELEMENTS 5 Claims, No Drawings [52] US. Cl 117/46, 264/0.5,1l7/l00 [51] Int. Cl G2lc 21/00 [50] Field of Search 264/05; 1 17/46 Primary Examiner-Carl D. Quarforth Assistant Examiner-S. Hellman Attorney-Larson and Taylor ABSTRACT: A method of producing coated particles of uranium dicarbide wherein a layer of pyrolytic carbon is deposited on uranium carbide particles of composition from UC, to UC at a temperature (e.g., lO0O C. to 1400 C.) such that little reaction occurs between the particles and the applied carbon coating followed by application of an outer coating of high density fission product retaining material on the carbon coated particles at a temperature (e.g., above 1450 C.) such that reaction takes place between the initial coating of pyrolytic carbon and the uranium carbide particles to form uranium dicarbide (UC A gap is produced between the particles and the coating on the particles.

NUCLEAR REACTOR FUEL ELEMENTS BACKGROUND OF THE INVENTION This invention relates to nuclear reactor fuel elements and in particular to that kind of nuclear reactor fuel element which comprises aggregated spheroidal particles of uranium carbide each particle having an outer coating of fission product retaining material, such as silicon carbide or high density pyrolytically deposited carbon.

U.S. Pat. No. 3,284,549 discloses a method for producing particles of uranium carbide having an outer coating of fission product retaining refractory material, comprising the steps of applying an initial thin coating of pyrolytic carbon on particles of uranium monocarbide at a deposition temperature such that very little reaction occurs between the applied coating of pyrolytic carbon and the uranium monocarbide particles, followed by application of an outer coating of fission product retaining refractor material on the pyrolytic carbon coated uranium monocarbide particles at a higher temperature such that reaction takes place between the initial pyrolytic carbon coating and the uranium monocarbide particles whereby the uranium monocarbide particles are converted to uranium dicarbide and voidage in the form of a gap is produced between the particles and the residual coating on the particles.

Coated particles in which the particles are directly bonded to the coating without the presence of a gap are subject to failure of the coating particularly on cooling ofthe coated particles from coating temperature to room temperature. On cooling of such coated particles to room temperature a considerable strain is developed in the coating because of the difference in thermal expansion coefficient between the particles and the coating, leading to cracking of the coating. Further an additional strain is imposed when the coated particles are reheated above the coating temperature and then cooled from this temperature in the form of final fabrication process which involves dispersion and incorporation of the coated particles into a dense matrix ofa refractory material in rod form.

Such failure can be avoided by making the particles of low density (i.e., containing a high amount of porosity) and by providing a thick coating on the particles. If the particles are sufficiently strong (i.e. thick enough) cracking due to thermal cycling is confined to the particles.

Coatings of for example silicon carbide and of sufficient thickness act as a barrier to both gaseous and metallic fission products which are generated during irradiation ofa fuel element incorporating such coated particles. Coatings of pyrolytically deposited carbon retain gaseous fission products and metallic fission products are retained by the dense matrix of nonfissile refractory material in which the coated particles are dispersed. Also the outer coating of the particles must be of sufficient thickness to withstand mechanical damage during the final fabrication ofa rod of dense matrix material in which the coated particles are incorporated as a dispersion.

The optimum thickness of outer coating required for the purposes of fission product retention and for resistance to mechanical damage during the final fabrication stage is, in general, less than the thickness of coating required to provide resistance to failure on thermal cycling.

ln coated particles produced by the method of U.S. Pat. No. 3,284,549 the presence of the gap between the particles and the outer coating permits thermal cycling of the particles without inducing radial cracking in the coating due to the larger thermal contraction of the coating as compared with the particles. Hence the method of U.S. Pat. No. 3,284,549 permits the production of particles having coatings of thickness optimized at about the minimum required for fission product retention and resistance to mechanical damage.

A coated particle produced by the method of U.S. Pat. No. 3,284,549 has a uranium content determined by three factors. These factors are the density of the uranium dicarbide kernel formed by conversion of the original uranium monocarbide particle by reaction with the initial layer of carbon, the dimension of the gap existing between the uranium dicarbide kernel and the residual coating on the particle after conversion, and

the thickness of the coating of fission product retaining material. The density which can be achieved in the uranium dicarbide kernel produced by conversion of the original uranium monocarbide particle is dependent on the density which can be achieved in the original uranium monocarbide particle. In general uranium monocarbide particles cannot be produced of density greater than percent of the theoretical density of uranium monocarbide. An outer coating of fission product retaining material is required of the optimum thickness necessary for fission product retention and resistance to mechanical damage. In the case of the gap which exists between the uranium dicarbide kernel and the coating after conversion of the original uranium monocarbide particles to uranium dicarbide the dimension of the gap is related to the amount of the initial layer of carbon which is required to convert the original uranium monocarbide particles to uranium dicarbide. The gap produced is not equivalent to the thickness of carbon which is used for conversion of the original uranium monocarbide particles as there is an increase in volume on conversion of uranium monocarbide to uranium dicarbide. However, it can be said that for a given particle size, the size of the gap produced, when starting with uranium monocarbide particles, will always be greater than a minimum value which is related to the amount of carbon required to convert the original uranium monocarbide particles to urani um dicarbide.

The factors of gap dimension and outer coating thickness determine the uranium concentration achieved in coated particles produced by the method of U.S. Pat. No. 3,284,549 in that these factors govern the final diameter of the coated particles.

For a particle having a given kernal diameter the higher is the final diameter of the particle the less is the uranium concentration in the particle. Having in mind that, irrespective of other factors, the thickness of the outer coating must be at least the optimum required for fission product retention and resistance to mechanical damage, it is the gap dimension which ultimately determines the uranium concentration achieved in particles having the optimum thickness of outer coating.

As stated above the method of U.S. Pat. No. 3,284,549 when starting with uranium monocarbide particles of given size, can only result in the production of coated particles having gaps which are larger than the minimum size determined by the amount of carbon required to convert the uranium monocarbide particles to uranium dicarbide. This imposes a limitation on the uranium concentration which can be achieved in coated particles produced by the method of the above numbered copending applications.

it is an object of the present invention to provide a modification of the method of U.S. Pat. No. 3,284,549 whereby the production of coated particles is enabled in which the gap produced between the uranium dicarbide kernel and the outer coating is less than the minimum size which can be achieved by the method of said United States Patent. Hence the method of the present invention produces coated particles having a higher uranium concentration than can be achieved by the method of U.S. Pat. No. 3,284,549 and it is a further feature of the present invention that coated particles can be produced having uranium dicarbide kernels of higher density than can be achieved by the method of U.S. Pat. No. 3,284,549 kernel density being the other factor which limits the uranium concentration which can be achieved in coated particles produced by the method ofthe United States Patent.

SUMMARY OF THE INVENTION According to the present invention a method for producing particles of uranium carbide having an outer coating of fission product retaining refrectory material comprises the steps of applying an initial coating of pyrolytic carbon to particles of uranium carbide of composition in the range VC to UC at a deposition temperature such that reaction between the applied coating of pyrolytic carbon and the particles of uranium carbide does not taken place by a substantial amount, followed by application of an outer coating on the pyrolytic car bon coated uranium carbide particles of a fission product retaining refractory material at a higher temperature such that reaction takes place between the initial coating of pyrolytic carbon and the uranium carbide particles to form uranium dicarbide (UC so that voidage in the form of a gap is produced between the particles and the coating on the particles.

Preferably the method is carried out starting with particles of composition from UC to UC,

The temperature of deposition of the initial coating of pyrolytic carbon is for example in the range lO0-1400 C. preferably 1l00 to 1300" C. The outer coating of fission product retaining refractory material is for example deposited at a temperature above 1450 C.

An initial coating of pyrolytic carbon may be deposited on the particles of greater thickness than required for conversion of the particles to uranium dicarbide so that during deposition of the outer coating of fission product retaining refractory material on the particles a proportion only of the initial coating of pyrolytic carbon is used up in conversion of the particles to uranium dicarbide and to produce the gap and a residual layer of the initial carbon coating is left inside the outer coating of fission product retaining refractory material which residual layer of carbon acts as a barrier against fission fragments from the uranium carbide kernel during irradiation.

The initial coating of carbon may be deposited on the particles by pyrolysis of methane or alternatively by the pyrolysis of acetylene.

Starting with particles of composition higher than UC e.g. of composition UC to UXC less carbon is required than in the case of particles of composition UC to effect conversion of the particles to uranium dicarbide. Thus the initial layer of carbon which is deposited can be less in thickness for particles having a higher carbon content than is required for particles of UC and a smaller gap is produced between the uranium dicarbide kernel and the outer coating of refractory material when subsequently deposited at a higher tem-- perature. Particles of composition UC to UC, when made by reaction sintering of a mixture of U0; and carbon can be made to 95 percent of the theoretical density whereas particles of UC can only be made to density of 90 percent theoretical. The use of particles of composition from UC, to UC, which are of 95 percent theoretical density in the method of the present invention results in the production of coated particles having uranium dicarbide kernels of higher density and therefore of higher uranium content than can be obtained starting with particles of composition UC, and of only 90 percent theoretical density.

\ DESCRIPTION OF THE PREFERRED EMBODIMENTS A method in accordance with the present invention will now be described by way of example.

Uranium carbide particles of spheroidal form are prepared by the method disclosed in U.S. Pat. No. 3,293,332. The method disclosed comses the steps of forming particles ofa mixture of uranium dioxide and a binder and reaction sintering the particles so formed so as to produce particles of uranium carbide. In the present case the relative amounts or uranium dioxide and carbon used are such as to produce particles of required composition in the range UC to UC The particles thus produced are graded by sieving to separate those of a size between for example 400 and 500 microns in diameter. Coating of the particles with pyrolytic carbon is now carried out in a fluidized bed reactor such as is also described in the above-mentioned U.S. Pat. No. 3,293,332. A bed of the particles is maintained in a fluidized condition in the reactor using high purity argon as the gas for fluidizati oi 1 A hydrocarbon gas such as methane is used for deposition of the pyrolytic carbon coatings. The methane is mixed with argon fluidizing gas stream.

An initial coating of pyrolytic carbon is deposited on the uranium carbide particles at a deposition temperature in the range I0OO C. to l400 C. for example at 1300 C. An initial carbon coating of at least sufficient thickness to react later with the uranium carbide particles and convert them to uranium dicarbide (UC is applied on the uranium carbide particles.

For example in the case of particles of composition UC UC UC and UC, having a density of percent of the theoretical density and having an applied initial coating of carbon density 1.5 grams/cmf, the following minimum thicknesses of initial carbon coating are required for conversion to UC TABLE 1 initial coating thickness required to convert particle to uranium di- Particle diameter carbide (UCms) The particle of composition UC is included for comparison purposes.

An outer coating of fission product retaining refractory material such as pyrolytic carbon or pyrolytically deposited silicon carbide is now deposited on the particles at a temperature above 1450" C. for example in the range l7502000 C. DUring deposition of the outer coating of fission product retaining refractory material the initial coating of carbon reacts with the particles to form uranium dicarbide (UC so that a gap is produced under the coating around the uranium dic'arbide particles.

The following table II shows the size of gap obtained for the particles of composition UC UC UC and UC in the table I above, it being the case that a thickness of carbon just sufficient as for conversion of the particles to uranium dicarbide is applied on the particles and an outer coating of fission product retaining material of 60 microns thickness is finally deposited on the particles.

The size of the gap produced depends on the amount of sintering which occurs in the particles on conversion to uranium dicarbide. However the gap will be between maximum and minimum values on the basis:

a. no reaction sintering occurs, i.e. there is no percentage change in theoretical density of the particle on conversion from the lower carbide to the dicarbide,

b. complete reaction sintering occurs, i.e. the particle is converted to percent dense uranium dicarbide.

It is probable that some sintering and densification occurs when the outer coating is applied particularly above l,500 C. and it is likely that the size of the gap will approach the maximum value in the following table.

TABLE ll Gap Gap Uranium dimension dimension content (micron) (microns) oi 500 Particle composition Particle Particle e micron expressed by diameter (a) (b) diameter (a) (b) coated pan Chemical formula (micron) Min. Max. (microns) Min. Max. grams/cm.

400 6. 8 I4 500 8. 8 18 4. 83 400 4. 7 11. 8 500 5. 35 14.4 4. 94 400 .2, 05 I 5 500 2. 48 11.5 4. D8 400 0. S 6. 85 500 1 8. 5 5. 00 400 l) 0 500 0 O 5.02

it can be seen from the above table ll that the uranium content of the coated particles increases as the carbon content of the original particle increases. The figures in respect of particles of composition UC are included for purposes of comparison but are not of practical significance because particles of composition UC, cannot be prepared of 90 percent of the theoretical density and in any case in such particles no gap exists between the kernel and the outer coating so that the outer coating will be subject to failure on thermal cycling. The advantage of increased uranium content levels out between particles of composition UC and UC and this is the preferred range of compositions.

In an alternative method an initial layer of pyrolytic carbon of greater thickness than required solely for conversion of the particles to uranium dicarbide can be applied on the particles. On application of the outer coating of fission product retaining refractory material at the higher temperature a proportion of the initial carbon coating will be used up in accordance, for example, with the thickness of carbon required for conversion of the particles to uranium dicarbide as given in table I and corresponding gaps will be produced as given in table II. However, a proportion of the nickel layer of carbon will remain as part of the coating of the particles forming a layer inside the outer coating of fission product retaining refractory materials Gap dimension assuming complete sintering of particle occurs on conversion to UCl-BS (microns) Particle composition expressed by chemical formula As the outer coating provides resistance to mechanical damage substantial reduction in the thickness of the outer layer of fission produce retaining material in order to obtain the same overall composite coating thickness may be undesirable.

in the case of particles 400 and 500 microns original diameter as disclosed in table land ll above, but having residual carbon layers for example of thickness in the range 25-50 microns it may be desirable for example that the outer coating of fission product retaining refractory material should have a minimum thickness of 50 microns giving overall coating thickness in the range 75 to 100 microns compared with the coating thickness of 60 microns used in the examples of tables I and ll above where the coating is solely of fission product retaining refractory material without the presence of a residual carbon layer. This of course means that the coated particle, will be of larger final diameter with a consequent reduction in uranium content.

The following table lll gives details of such coated particles of 500 microns original diameter, having coating including a residual carbon layer of 25 microns in thickness and having an outer coating of fission product retaining refractory material 50 microns in thickness (i.e. total coating thickness 75 microns).

TABLE III.

Thickness of initial carbon coating deposited from methane (density 1.5 grams/ cc.) (microns) Thickness of initial carbon Thickness coating used in of residual conversion of carbon layer particle to in final coat L'C (microns) (microns) Uranium content of coated particle grams/cm.

separated by the gap from the kernel of the particle. By way of example in the case of particles as disclosed in table II, if the coating of fission product retaining refractory material plus the residual carbon coating is made of 60 microns thickness the same uranium content will be obtained in the particles.

If the initial carbon layer is'deposited by pyrolysis of acetylene instead of by pyrolysis of methane the carbon deposited is of lower density i.e. the density of carbon deposited by pyrolysis of methane is 1.5 grams per cubic centimeter whilst the density of carbon deposited by pyrolysis of This of course will require the deposition of a thinn r out r 40 acetylene is 1.0 to 1.2 grams per cubic centimeter. lfacetylene coating of fission product retaining refractory aterial on th is used to deposit on initial carbon layer of sufficient thickness particles to achieve the same final overall coating thickness. to lea e a residual in e layer of carbon in the coating after The existence of the residual inner layer of carbon in the Conversion Ofthe particles 10 Las this residual Carbon ye coated particles provides a barrier against escape of fission will be of lower density than a residual carbon layer left from products generated in the kernel during irradiation. For this an n i r n coati g h h h n d p s ted by he purpose the residual carbon layer should have a thickness of pyrolysis of methane. The lower density carbon layer about 25 to microns and to provide an ov rall oati g deposited from acetylene provides a more efficient barrier to thickness of 60 microns an outer coating of fission produce fi n Pr th n 1 5 a higher density Car on laye as retaining refractory material of related thickness in the range 50 deposited from h 35 to 10 microns must be applied on the particles. However, as an initial carbon layer deposited by pyrolysis of To obtain a required thickness of residual carbon in the acetylene is of low density for example l gram per cubic cencoating an initial carbon layer of thickness e ual to the timeter a greater thickness of carbon will have to be provided amount of carbon required to convert the particles to uranium in the initial yer for Conversion 0f the Particles l0 UCL85 than dicarbide plus the required residual thicknessisapplied on the is th Case when the initial carbon layer is deposited from particles. methane.

For example in table I above the required thickness of car- T bl IV b l ho th o ari n b tween th bon required for conversion of 400 micron particles of COITI- thickness of initial coating required to convert 500 micron position UC to UC is in the range 6 to 15 mi lhefediameter particles of the indicated composition (a) when the fore the thickness of the initial carbon coating required on initial carbon coating is deposited by pyrolysis ofmethane and such particles for production of coa ed parti les h ing (b) when the initial carbon coating is deposited by pyrolysis of coatings with a residual Carbon layer in he r ng 2 I 50 acetylene. In the case of deposition of the initial carbon layer mi n ll be 31 10 40 microns o produce the minimum 25 from acetylene a greater thickness of carbon is required for micron residual carbon layer) and 56 to microns (to 6 conversion and this ofcourse is reflected in the size ofthe gap produce the maximum 50 micron residual carbon layer). 5 which is produced.

TABLE IV (a) (a) (b) (b Initial carbon initial carbon coating thickness coating thickness required for required for conversion when conversion when deposited from deposited from methane (carbon acetylene (carbon density 1.5 density 1.0 Particle composition grams per cubic Gap grams per cubic Gap expressed by chemical centimetre) dimension centimetre) dimension formula (microns) (microns) (microns) (microns) UC1 2 18.75 14. 4 27 23. 5 UCl.l. 12.5 11.5 18 16,5 7 8. 5 10 11.0

In the method for fabrication of uranium carbide particles as disclosed in U.S. Pat. No. 3.293332 particles of a mixture of uranium dioxide and carbon are grown to a desired size and then reacted and sintered in vacuum in a two temperature process. At the lower temperature, about 1,400 C., UO is reduced by carbon to form uranium carbide and when reaction is complete, sintering is effected at the higher temperature, l600-l700 C. The carbotherrnic reduction occurs according to the chemical equation If the carbon/U ratio in the original mix is that given by the above equation, the final product is essentially uranium monocarbide. If the carbon content is at a higher value then, in general a two phase product will result comprising uranium mono and di-carbide phases, the proportions depending on the original carbon concentration. Eventually the final product will consist entirely of dicarbide although the carbon content of this phase is variable and always lower than that defined by the formula, UC In general in preparations made by reduction of U0 the carbon cohtent may be expressed by the formula UC It is also the case that if the CO evolution is not maintained at a steady rate and some sintering is allowed to occur at the reaction temperature, which acts to slow down the reduction reaction, then it is possible to nucleate also the sesquie carbide phase (U C It is a feature of the above process that where UC is the intended final product it is difficult to remove all oxygen completely because of the lack of ideal homogeneity of the mix and the difficulty of removing all CO before sintering is effected. Thus, the final product, although single phase, will contain between 0.1 wt. percent and 0.2 wt. percent oxygen and the final sintered density is limited to a maximum value of 12.3 g./cm.", i.e. 90 percent theoretical density. When the carbon content is increased to the level where the final product may be expressed by the formula UC the oxygen content falls to 0.05 wt. percent and at a final composition of UC oxygen contents 5O p.p.m. may be obtained. In addition this complete reduction at the reaction temperature allows a product of higher relative density. For example UC gives a 95 percent relative density at 12.0 g./cm. Above UC the density of the final product falls rap id ly until at UC, relative densities between 60 percent and 70 percent represent the maximum value. The associated high porosity of the samples leads to extensive oxidation under atmospheric conditions and the oxygen content of the product rises again to high values.

As it is possible to prepare particles of 95 percent of the theoretical density in the composition range UC to UC even greater uranium contents can be obtained than as shown in the tables above which relates to particles having a density of percent of the theoretical density. Thus for the composition UC, at a density of 12.0 grams/cm. percent of the theoretical density) the uranium density of the final coated particle (500 microns original diameter) calculated on the same assumptions is 5.25 grams of uranium per cmf, which is 8 percent greater than can be obtained starting with a particle of composition UC We claim:

1. A method for producing particles of uranium carbide having an outer coating of fission product-retaining refractory material comprising the steps of applying an initial coating of pyrolytic carbon to particles of composition in the range UC to UC at a deposition temperature in the range 1000 C. to 1400 C. such that reaction between the applied coating of pyrolytic carbon and the particles of uranium carbide does not take place by a substantial amount followed by application of an outer coating on the pyrolytic carbon-coated particles ofa fission product-retaining refractory material at a higher temperature above 1450 C. such that reaction takes place between the initial coating of pyrolytic carbon and the uranium carbide particles to form uranium dicarbide (UC Q so that voidage in the form ofa gap is produced between the particles and the coating on the particles.

2. A method as claimed in claim 1 wherein the uranium carbide particles employed are of composition from UC, to ur 3. A method as claimed in claim 1 wherein an initial layer of carbon is deposited on the uranium carbide particles of greater thickness than required for conversion of the uranium carbide particles to uranium dicarbide whereby a residual inner layer of carbon remains as part of the coating on the particles.

4. A method as claimed in claim 1, wherein the initial carbon coating is deposited on the uranium carbide particles by pyrolysis of methane.

S. A method as claimed in claim 1, wherein the initial carbon coating is deposited on the uranium carbide particles by pyrolysis of acetylene. 

2. A method as claimed in claim 1 wherein the uranium carbide particles employed are of composition from UC1.4 tO UC1.6.
 3. A method as claimed in claim 1 wherein an initial layer of carbon is deposited on the uranium carbide particles of greater thickness than required for conversion of the uranium carbide particles to uranium dicarbide whereby a residual inner layer of carbon remains as part of the coating on the particles.
 4. A method as claimed in claim 1, wherein the initial carbon coating is deposited on the uranium carbide particles by pyrolysis of methane.
 5. A method as claimed in claim 1, wherein the initial carbon coating is deposited on the uranium carbide particles by pyrolysis of acetylene. 